Imaging description

Medical imaging evaluation of hip dysplasia or acetabular maldevelopment is primarily done by radiography. Strict attention to detailed technique is essential for accurately measuring the degree or severity of hip dysplasia. The most commonly used view is the AP radiograph of the pelvis. It should be done in the standing position with approximately 20° of internal rotation of the lower limbs. The focus film distance should be 100 cm. The imaging criteria for acceptance of a radiograph to evaluate for hip dysplasia are symmetric appearance of the obturator foramina, symmetric appearance of the iliac crests, and near superimposition of the coccyx with the symphysis pubis. Common measurements obtained on the pelvic AP radiograph are the center-edge angle, the horizontal toit externe angle, and the percentage of femoral head coverage. Some of these measurements require identification of the most medial point of the weight bearing acetabulum. The weightbearing portion of the acetabulum is identified by its sclerotic, arched appearance known as the sourcil (eyebrow) (Figure 51.1).

The second most commonly used radiographic view for measuring the degree of acetabular dysplasia is the false profile view. This view is obtained with the patient in the standing position with the pelvis rotated 65° relative to the film or detector. The imaging appearance criteria for acceptance of a false profile image as adequate for measuring acetabular dysplasia is if the distance between the two femoral heads is approximately the size of one femoral head. The vertical-center-anterior edge angle is measured on the false profile view (Figure 51.2). This view may also show early signs of osteoarthritis (small osteophytes and/or mild joint space narrowing) that is not yet seen on the AP view of the pelvis.